The invention relates to a method of measuring a time difference, the method being used in a radio system comprising at least two base stations and a terminal, in which radio system an observed time difference OTD of the base stations is measured and a geometrical time difference GTD is calculated.
In known radio systems timing of a signal coming from a transmitter to a receiver can be measured particularly for the purpose of locating a terminal. In a terminal location system based on measuring a signal of a base station, the terminal measures an observed time difference OTD of the arrival of signals of at least two, or preferably at least three, base station pairs. By assuming that the base stations transmit synchronously, i.e. the same signals leave the base stations simultaneously, the observed time difference at reception of the signals which arrive from the two base stations corresponds to the distances of the base stations, i.e. the observed time difference is a geometrical time difference GTD. In that case, as the location of the base stations is typically fixed and known, hyperbolas can be formed by means of the differences in signal propagation times, and the terminal is to be located at the intersection of the hyperbolas on the basis of the measurements. A drawback of such a measurement is that the transmissions of the base stations are not completely synchronous.
Signals of base stations can be measured by means of time difference units which have a known location and can be used for determining a real time difference of the transmissions of the base stations. The RTD in question can be further utilized when the terminal measures the observed time difference OTD of the base stations, since the real time difference RTD of the base stations can be subtracted from the observed time difference OTD measured by the terminal. However, since the observed time difference OTD measured by the terminal and the real time difference RTD of the base stations, measured by the time difference unit, are measured at different times due to a delay created by a data transmission channel, for example, the measurement of the geometrical time difference GTD still contains errors, which brings about inaccuracy in the location determination of the terminal.
An object of the invention is thus to provide a method of measuring a time difference and a radio system implementing the method so as to solve the above problems associated with the accuracy of the time difference.
This is achieved with a method of the type presented in the introduction, which is characterized in that the real time difference RTD of the base stations is formed by means of the observed time difference OTD and the geometrical time difference GTD as a function of time by connecting the moment of occurrence of the measured real time difference RTD to the real time difference RTD of the base stations.
The invention also relates to a radio system comprising at least two base stations and a terminal, a time difference unit arranged to measure an observed time difference OTD of the base station, and a time difference centre whereto the time difference unit is arranged to transmit its measuring information, and which radio system has information available on geometrical time differences GTD of the base stations. The radio system is characterized in that the time difference centre is arranged to form a real time difference RTD of the base stations as a function of time by utilizing the observed time difference OTD and the geometrical time difference GTD calculated in a manner such that the time difference centre is arranged to connect the moment of occurrence of the real time difference RTD to the real time difference RTD.
The method and system of the invention provide many advantages. The measurement of the real time difference RTD and the observed time difference OTD measured by the terminal can be matched in time, thereby making it possible to reduce the error caused by the real time difference RTD. Hence, the terminal can be located more accurately as compared with the prior art.